1. Field of the Invention
This invention relates to the field of graphics display with data processing systems. More particularly, this invention relates to graphics clipping.
2. Description of the Related Art
It is known to provide graphics displays comprising a number of overlapping areas, such as the rectangular windows in the Operating System/2 (trademark) or Graphical Data Display Manager programs produced by International Business Machines Corporation. Each window contains graphics objects and as the user varies the size of the window the system determines what portion of the objects are inside the window and displays only that portion. This process is known as clipping. The commonly assigned application of applicant J. A. Herrod, Ser. No. EP 91306143.8, filed in the European Patent Office on Jul. 5, 1991, concurrently herewith and entitled "Graphics Processing Method and Apparatus" (Attorney Docket No. UK9-90-008), describes a number of clipping techniques.
In such displays each window can be thought to be assigned a priority, with windows of higher priority overlying windows of lower priority where they overlap. One known way of dealing with the overlapping windows is to start from the window of lowest priority, draw this and then proceed to draw the window with the next highest priority until all the windows have been drawn. By proceeding in such a way those portions of the lower priority windows that are overlapped by higher priority windows are first drawn and then overlaid.
Such a technique is disadvantageously slow and distracting for the user. Time is spent needlessly drawing portions of low priority windows that will subsequently be overlaid by the higher priority windows. Updates to an overlapped window require all the overlapping windows to be redrawn. In addition, such a screen cannot be readily printed or plotted on a printer or plotter since it is not possible to wipe out overlapped parts as you can on a screen.
An alternative way of dealing with overlapping windows would be to provide a clipper which assigns ownership of picture elements of a display to a particular window and only allows the window owning a given picture element to write to it. Such a technique requires a disadvantageous processing and data storage overhead resulting from the need to deal with each picture element separately. The relative inefficiency of this technique is particularly apparent when graphics orders (i.e. instructions defining graphics objects such as lines, areas, circles etc.) are being processed.
It will be seen that there exists a need for way of dealing with overlapping windows (areas of overlapping graphics--not necessarily rectangular or variable in size) which does not rely on first drawing and then overlaying the lower priority windows and yet does not introduce the disadvantages associated with the alternative.